Method and apparatus for direct electrothermal-physical conversion of ceramic into nanopowder

ABSTRACT

A process for synthesizing nanosized powders utilizes a hybrid exploding wire device containing a solid metal wire fuse in the bore of a tube that is open at each end. The ends of the fuse are connected to electrodes on the ends of the tube. The electrodes are designed to erode to maintain a heavy metal plasma. The bore may comprise a corresponding ceramic to be produced, and a microcrystalline powder of a corresponding ceramic may be retained within the bore. An electrical discharge vaporizes and ionizes the fuse. The tube confines the radial expansion of the plasma such that the plasma exits from both ends of the tube where it reacts with a suitable gas to form nanoscale particles. In addition, the plasma gas ablates and vaporizes a portion of the bore wall to contribute to the nanoceramic synthesis. Other alternatives include replacing the fuse with a thin conductive sheath or a consumable metal insert.

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/396,046, filed Sep. 15, 1999, entitled “Method and Apparatusfor Direct Electrothermal-Physical Conversion of Ceramic intoNanopowder,” which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a method and system for theproduction of submicron materials, and in particular to a method andsystem of synthesizing, in bulk quantities, nanosized powders, includingnanocrystalline ceramics. Even more particularly, the present inventionrelates to a method and system for increasing the rate and therebyreducing the cost of the production of bulk quantities of nanosizedpowders by electrothermal gun synthesis.

2. Description of Related Art

Ceramic materials are used in a wide variety of applications, andgenerally have excellent heat resistance, corrosion resistance, andabrasion resistance, as well as unique electrical and opticalproperties. Ceramic material, as used herein, generally refers to anoxide, nitride, boride or carbide of a metal, or a mixture thereof. Veryfine ceramic powders are used in a large number of industrial processesto introduce or modify material properties. These materials can posedifficulties in sintering but, when they are converted to ultrafineparticles, particularly submicron crystalline particles, numeroustraditional problems are avoided. Accordingly, several processes havebeen devised for fabricating ultrafine, or submicron, crystallinematerials, such as those of 1-500 nanometer size, referred to herein as“nanosized,” “nanocrystalline,” “nanoparticles,” and the like.

Techniques for producing nanocrystalline materials generally fall intoone of three categories, namely, mechanical processing, chemicalprocessing, or physical (thermal) processing. In mechanical processes,fine powders are commonly made from large particles using crushingtechniques such as a high-speed ball mill. There are severaldisadvantages with this approach.

Sometimes metallic powders and highly reactive metals are combined withand subjected to such milling, which can pollute the material with ananocrystalline alloy. Fragmented powders produced by mechanicalprocesses can also result in particles of inconsistent shapes and sizes,and are often coarse and so not suited for high-performanceapplications.

With chemical processes, nanocrystalline materials are created from areaction that precipitates particles of varying sizes and shapes, usinga family of materials known as organometallics (substances containingcombinations of carbon and metals bonded together). It is difficult,however, to produce ultrafine ceramics using organometallics withoutintroducing excess carbon, or nitrogen (or both) into the finalcomposition. Solution-gelation (sol-gel) ceramic production is similarto organometallic processes, but sol-gel materials may be either organicor inorganic. Both approaches involve a high cost of raw materials andcapital equipment, limiting their commercial acceptance.

One of the earliest forms of physical, or thermal, processing, involvesthe formation and collection of nanoparticles through the rapid coolingof a supersaturated vapor (gas phase condensation). See, e.g., U.S. Pat.No. 5,128,081. In that example, a raw metallic material is evaporatedinto a chamber and raised to very high temperatures, and then oxygen israpidly introduced. See also U.S. Pat. No. 5,851,507, in which a carriermedium is mixed with precursor material which is vaporized andsubsequently rapidly quenched.

Thermal processes create the supersaturated vapor in a variety of ways,including laser ablation, plasma torch synthesis, combustion flame,exploding wires, spark erosion, electron beam evaporation, sputtering(ion collision). In laser ablation, a high-energy pulsed laser isfocused on a target containing the material to be processed. The hightemperature of the resulting plasma (greater than 10,000° K.) vaporizesthe material so quickly that the rest of the source (any carrier andquenching gases) can operate at room temperature. The process is capableof producing a variety of nanocrystalline ceramic powders on thelaboratory scale, but it has the great disadvantage of being extremelyexpensive due to the inherent energy inefficiency of lasers, and so itnot available on an industrial scale.

The use of combustion flame and plasma torch to synthesize ceramicpowders has advanced more toward commercialization. In processes, theprecursor material can be a solid, liquid or gas prior to injection intothe flame or torch, under ambient pressure conditions (the most commonprecursor state is a solid material). The primary difference between thetwo processes is that the combustion flame involves the use of anoxidizing or reducing atmosphere, while the plasma torch uses an inertgas atmosphere. Each of these processes requires relatively expensiveprecursor chemicals, such as TiCl₄ for the production of TiO₂ by theflame process, or TiC and TiB₂ by the plasma process. A feature of bothmethods is the highly agglomerated state of the as-synthesizednanocrystalline ceramic powders. While for many applications theagglomeration of the powders is of little significance, there aresituations where it is a shortcoming. Loosely agglomerated nanoparticlepowders are produced in the combustion flame method of U.S. Pat. No.5,876,683.

In the plasma process, reactants or feed materials are delivered to aplasma jet produced by a plasma torch. See generally, U.S. Pat. Nos.4,642,207 and 5,486,675. Alternatively, the feed material may bedelivered to the plasma stream by arc vaporization of the anode. Theanode is normally metallic but may be a metal-ceramic composite. Animproved plasma torch process is described in U.S. Pat. No. 5,514,349.That process can produce non-agglomerated ceramic nanocrystallinepowders starting from metalorganic precursors, and uses rapid thermaldecomposition of a precursor/carrier gas stream in a hot tubular reactorcombined with rapid condensation of the product particle species on acold substrate. Plasma torch processes, while gaining some limitedcommercial acceptance, are still energy inefficient and often involvematerials which are extraneous to the products being produced. Forexample, in the '349 patent, a working gas must be heated by the plasmaarc, which is wasted energy. Also, since the product particles aresuspended in the hot process gas stream, it is necessary to quench notjust the particles but the process stream as well. The multiple gasesused (the reaction gas, quench gas, and passivating gas) are eitherwasted, or must be separated for reuse.

Another apparatus for producing nanosized particles is the explodingwire device. A conventional exploding wire device is illustrated in FIG.5. A pulsed current discharge is driven through a small diameter wirethat is typically on the order of 0.1 mm. The resulting joule heating ofthe wire vaporizes and ionizes the wire resulting in plasma whichexpands radially. The nearly cylindrical plasma contact surface whichexpands into the ambient gas, undergoes a reaction with the gas andsubsequently cools. The mixing process relies on molecular diffusion andthe cooling process is nearly isentropic. Production rates are typicallyonly a few milligrams owning to the small diameter wire necessary forcomplete vaporization.

Another device that builds on the exploding wire method is aconventional capillary plasma device, shown in FIG. 6. Conventionalcapillary plasma devices produce hydrocarbon-based plasmas that are usedto ignite explosives because the plasma will not negatively react withthe explosive. The capillary plasma device consists of two non-erodingelectrodes positioned at the ends of a non-conducting bore with one openend. A fuse wire is connected between the electrodes. The process beginswith an electrical discharge that explosively vaporizes and ionizes thefuse wire between the two electrodes. The discharge is maintained by theerosion and subsequent ionization of the liner to produce a dense plasmainside the bore which then exits from the open end of the device. Thesealed end or breech of the gun must be capable of withstandingextremely high pressures in the order of 10,000 psi. The plasma exitsthe gun and is then used to ignite the explosive. The length to diameterratio of bore, L/D, is large which helps to promote bore wall erosion tosustain the discharge. This process is not well suited for ceramicproduction because the erosion of a ceramic bore would not produce theionization needed to sustain the plasma and consequently the productionwould be low.

The electrothermal gun, also known as an electrogun or capillarydischarge device (shown in FIG. 7), is a pulsed power device for theproduction of very high velocity plasma jets and vapors of differentmetals such as aluminum or titanium. A pulsed, high current arc isstruck down the barrel of a gun, between an electrode in the breech ofthe gun and an electrode at the muzzle. This arc produces rapidvaporization of the electrodes and of the barrel of the gun so that apulsed, high-temperature, high-velocity plasma jet is fired out themuzzle.

Electrothermal guns are superficially similar to plasma torches. In aplasma torch, an electric arc is used to convert a gas stream into aplasma jet. Electrothermal guns have been used for a variety ofapplications such as laboratory plasma producers, spacecraft thrusters,propellant ignitors for high performance guns, and plasma armaturesources for electromagnetic rail guns. Electrothermal guns also havebeen used to vaporize and project an initially liquid gas (such asargon) to atomize an external stream of molten metal. In addition,electrothermal guns have been used to heat and propel powdered materialfor the application of coatings. “Electrothermal gun synthesis” is theuse of an electrothermal gun to synthesize nanomaterials such aspowdered material with particle sizes of approximately 100 nanometers orless. The plasma jet itself is converted physically or physically andchemically into ultra-fine powder. The material composing the plasma jettypically comes from the breech electrode or the muzzle electrode.

In operation, it is frequently necessary to change the breech electrodeof the gun. For production of bulk quantities, it is an economicnecessity to accomplish this change automatically rather than manually.Thus, simplification of the breech electrode replacement processproduces significant cost reduction. For proper operation, the breechelectrode must form an effective seal against the reverse flow ofhigh-pressure plasma. The breech electrode is installed with ahigh-force mechanical preload to insure that the seal remains effectiveagainst the sudden onset of high-pressure plasma. In addition, aheavy-duty electrical connection to the breech electrode must beeffected. A final consideration is that when the breech electrode ischanged, the stub end of the breech electrode is wasted. A method andsystem for increasing the production rate and reducing the cost of theproduction of bulk quantities of nanosized powders by electrothermal gunsynthesis is needed to overcome these limitations.

SUMMARY OF THE INVENTION

The limitations of the prior art are overcome by a new process for thesynthesizing nanosized powders. The process utilizes a “hybrid explodingwire” (HEW) device containing a small diameter solid metal fuse wire orfoil sheath inside a cylindrical tube. The ends of the fuse areconnected to electrodes which are designed to have sufficient erosion tomaintain the heavy metal plasma. The cylindrical tube is designed tocontain the large pressures generated when the plasma is produced. Thebore may be made of but is not limited to the corresponding ceramics tobe produced. Microcrystalline powder of corresponding ceramic may alsobe retained within the bore.

The process is initiated with an electrical discharge that vaporizes andionizes the fuse. The tube confines the radial expansion of the plasmawhich forces it to exit from both ends of the tube where it mixes andreacts with a suitable gas to form nanoscale particles. In addition, theplasma gas will ablate and vaporize the bore wall contributing to thenanoceramic synthesis.

The benefits of the current invention are both chemical, via reaction ofthe metal vapor with the reaction gas, and physical via ablation of thebore wall, conversion processes are employed to produce thenanocrystalline ceramics. Having the plasma exit both ends of the boreeliminates the problems associated with high pressure breech seals andeliminates the corresponding forces because the internal pressures areused to balance each other. The dual bore also allows increasedproduction by simultaneously operating two guns.

In an alternate embodiment, the fuse wire is replaced with a thinconductive sheath. The conductive sheath fits inside the bore which hasbeen undercut to form an annular area. The annular area is filled withmicron sized particles of the desired compound, reactant metal, orcombination thereof. The process is initiated as described earlier andthe conductive sheath is instantly vaporized. The micron particles areexposed to the plasma where they are vaporized and ejected with theplasma. The plasma jet with the ceramic vapor is rapidly cooled andnanosized particles are formed. The large surface area to volume ratioof the microparticle material overcomes the problems with low productionrates by allowing quick and efficient conversions to nanoparticleceramics.

In another embodiment, a consumable metal insert is placed in the centerof the bore. In this version, a spark progresses from one electrode tothe metal insert, and then from the insert to the other electrode. Themetal insert is eroded and/or vaporized to sustain the discharge andincrease production of the nanosized material. The external electrodescan be made of tungsten in an effort to minimize erosion so that they donot have to be replaced. Because the metal insert is in the center, theforces on it are symmetrical and they will balance one another. Thiseliminates the need for high pressure seals. In addition, there is noelectrical contact on the metal insert so that it can be replacedquickly and easily, thus facilitating higher rates of production overprior art methods.

It is one object of the present invention to expedite changing thebreech electrode of an electrothermal gun.

It is another object of the present invention to eliminate materialwaste by providing for complete consumption of the breech electrodewithout leaving a stub or heel.

The foregoing objects are achieved by modifying the design of theelectrothermal gun to simplify installation of the breech electrode. Themodified design is open at both ends, and the breech electrode isreplaced by a central electrode. In operation, two opposingelectrothermal jets are produced, one exiting from each of the two openends of the gun. There are no mechanical electrical (i.e., direct)connections to the central electrode. Current is transferred to thecentral electrode by two arcs, one arc attaching to each end of thecentral electrode. There is little or no pressure differential betweenthe two ends of the central electrode, so the need to provide a seal iseliminated. Also, if a pressure differential does develop between thetwo ends of the central electrode, there is no adverse consequence tothe leakage of plasma and molten metal across the central electrode.

The above as well as additional objectives, features, and advantages ofthe present invention will become apparent in the following detailedwritten description.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the present invention areset forth in the appended claims. However, the invention itself as wellas a preferred mode of use, further objectives, and advantages thereof,will best be understood by reference to the following detaileddescription of an illustrative embodiment when read in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a schematic diagram of a system for electrothermal synthesisof nanocrystalline ceramics;

FIG. 2 is a cross-sectional view of an electrothermal gun used with thesystem of FIG. 1;

FIG. 3 is a pictorial representation of the synthesis of nanocrystallineceramic powder using the system of FIG. 1;

FIG. 4 is a cross-sectional view of an alternative electrothermal gun;

FIG. 5 is a schematic drawing of a prior art exploding wire device.

FIG. 6 is a schematic drawing of a prior art capillary plasma device.

FIG. 7 is a schematic drawing of a prior art electrothermal gun.

FIG. 8 is a schematic diagram of a hybrid exploding wire device havingdual bores constructed in accordance with a first embodiment of thepresent invention and showing an initial processing stage.

FIG. 9 is a schematic diagram of the hybrid exploding wire device ofFIG. 8 showing an intermediate processing stage.

FIG. 10 is a schematic diagram of the hybrid exploding wire device ofFIG. 8 showing a final processing stage.

FIG. 11 is a schematic diagram of a second embodiment of a hybridexploding wire, dual bore device having a conductive insert and isconstructed in accordance with the invention.

FIG. 12 is a schematic diagram of a third embodiment of a hybridexploding wire, dual bore device having an encapsulated insert and isconstructed in accordance with the invention.

FIG. 13 is a schematic diagram of a fourth embodiment dual bore devicehaving a central electrode and is constructed in accordance with theinvention.

FIG. 14 is a schematic diagram of the dual bore device of FIG. 13 inoperation.

FIG. 15 is a schematic illustration of an automated reloading mechanismfor the dual bore device of FIGS. 13 and 14.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference now to the figures, and in particular with reference toFIG. 1, there is depicted one embodiment 8 of an electrothermal systemfor synthesizing ceramic nanopowders constructed in accordance with thepresent invention. System 8 is generally comprised of a high-currentelectrical power supply 10 with heavy-duty wiring 12 for conducting anenergetic current pulse, an arc initiator power supply 14 with wiring16, a ceramic electrothermal gun (electrogun) 18 with a cooling system20, a reactor atmosphere supply system 22 with a supply pipe 24 andatmosphere control system 26, and a reactor vessel 28 having a reactoratmosphere 30 illustrated with suspended nanoparticles 32, and a layerof settled nanopowder 34 on the floor of the vessel. As explainedfurther below, power supply 10 provides pulsed current to electrogun 18in concert with initiation of an arc by initiator power supply 14, whichresults in activation of electrogun 18. A plasma stream from electrogun18 entrains raw metal precursor material and ceramic precursor materialwhich become vaporized in reactor vessel 28, and subsequently condenseas nanocrystalline particles 32.

Referring now to FIG. 2, electrogun 18 may be constructed in a fashionsimilar to conventional electrothermal guns (such as those used forspacecraft thrusters, the production of railgun plasma armatures, or theignition of propellants to accelerate projectiles in guns), except thatelectrogun 18 is provided with a ceramic barrel, that is, a barrel whosematerial is the same (chemically, although not in the same physicalstate) as the nanopowder which is desired to be produced. In anillustrative embodiment, electrogun 18 includes a cathode 40, anon-eroding anode 42, a structural shell or housing 44 with coolantchannels 60, a ceramic liner 46 forming the gun barrel, a muzzle seal48, a breech seal 50, and arc initiator lines 52.

In contrast with prior art electroguns, the material of ceramic liner 46is specifically selected to erode during generation of the plasma streamwithin the bore of electrogun 18. The synthesis process thus preferablyincludes the generation of nanosized particles from both (1) thereaction of the metallic (or organometallic) cathode 40, and (2) thephysical conversion of the material of ceramic liner 46 to a nanosizedform as a result of the gun blast. Preferably, electrogun 18 has alength-to-diameter ratio of at least ten.

The synthesis process is illustrated further in FIG. 3. Power issupplied to cathode/anode pair 40/42 via power supply 10 while anelectric arc is established via initiator lines 52. The high-currentelectric arc 80 passes between cathode 40 and anode 42, and ahigh-pressure, high-velocity, high-temperature stream of plasma 82 flowsdown the bore of electrogun 18. Ceramic material 47 is ablated fromceramic liner 46, and become entrained in plasma stream 82. Particlesthus entrained lose mass through vaporization, and become smaller orvaporize completely. Reactant material 39 from cathode 40 also becomesentrained in plasma stream 82.

As the high-pressure plasma exits the confines of electrogun 18, itundergoes rapid isotropic expansion. One result of the rapid expansionis a rapid cooling. The cooled plasma then condenses into ahigh-velocity spray of extremely fine (nanosize) ceramic particles 84.The energetic expansion produces turbulent mixing of the condenseddroplets or particles with the reactor atmosphere 83. Any metallicparticles 86 produced by electrode erosion or by disassociation ofceramic quickly react with the reactor atmosphere 83, forming ceramicparticles 84. Thus, a suspension of nanoparticles is produced, whichgradually settle to the floor of the reactor vessel where they may becollected.

The reactor atmosphere serves two primary purposes, to react any metalparticles which may be mixed in with the ceramic particles, and torapidly quench the ceramic particles, since unquenched particles wouldtend to bond tightly together or even grow together into a singleparticle. Quenched particles may stick together, but more loosely thanhot particles. Quenched particles do not tend to grow into a singleparticle.

Accordingly, the electrothermal synthesis taught herein provides amethod for the direct and efficient conversion of ceramic material intoceramic nanopowder, thereby realizing a material cost saving incomparison to competing methods. Energy costs are also reduced inasmuchas the ceramic feed material is heated directly rather than indirectlyas is the case of prior art plasma torch processes. The presentinvention, unlike plasma torch processes, requires no working gas. Thereis no mixing of gas streams, and no circulation of the reactoratmosphere through the plasma arc, and further there is no need to use arefrigerated quenching surface. Reactions go to completion quickly. Thetechnique has proven particularly suitable for production of titaniumand aluminum oxide and nitride. No byproducts are produced, and theprocess is well-suited for automation.

FIG. 4 illustrates an alternative embodiment for an electrogun 70 whichmay be used with the present invention. Electrogun 70 has a conical,rather than cylindrical, bore. Additionally, a ceramic insert 72 havinga cylindrical body 74 and a conical tip 76 is advanced into the bore.The conical bore and conical tip 76 form a divergent annular passageway.Ceramic material which is to be physically converted to nanopowder isextracted from both the ceramic bore liner and insert 72. In thismanner, the insert is easily changed when it has been consumed (i.e., itis used for more than one shot of electrogun 70). The replacement ofinsert 72 is particularly advantageous since it is more easily erodedthan the bore liner, and the liner is less conveniently replaced. Thecross-sectional area of the annular passageway is easily adjusted bychanging the axial (longitudinal) position of the insert, so simpleadjustments compensate for erosion of the conical bore liner as well (tomaintain a particular passageway cross-section). Erosion of thepassageway is actually self-adjusting, since erosion will be greaterwhere the passageway is smaller, and vice-versa. Physical properties ofthe insert can be adjusted to favor erosion of the insert. For example,it can be made relatively porous.

In another embodiment, the ceramic precursor material may be injected asparticulates into the plasma stream, wherein the ceramic precursorparticulates have a first size (e.g., micron or larger), and the ceramicparticles suspended in the vessel have a second size which issubstantially smaller than the first size (e.g., nanosized). Theprecursor material would preferably be injected radially in the breechregion, allowing sufficient residence time within the gun. The injectiontechnique may be combined with the above-described technique using theceramic liner 46 which erodes during generation of the plasma stream.

FIGS. 8-10 provide step-by-step, schematic diagrams of a new processusing a preferred embodiment of the present invention that is configuredto produce nanocrystalline alumina (Al₂O₃). A thin metal wire or sheath110, in this case aluminum, is placed between the cathode 112 and anode114 in the bore 120 of a reactor vessel having at least two exit ports.Sheath 110 acts as a fuse that initiates a discharge. The fuse is thesame material as the cathode and anode, so as to eliminatecontamination. The cathode and anode are positioned on each end of abore 120. The bore is preferably made from the same material as thatwhich is being produced and is designed to contain the pressures thatare produced during the process. The bore may contain microparticles 122of the material to be produced. In the current invention, themicroparticles are positioned within an annular volume 124 between thebore and the fuse and held in place by the fuse sheath. In alternateembodiments, the material may be contained in a thin tube placed withinthe bore having a fuse wire passing through the tube to connect bothelectrodes. The microparticles also may be pressed into the cathode andanode.

As shown in FIG. 8, the process begins by discharging an electricalcurrent from a capacitor bank 126, between cathode 112 and anode 114.When the discharge is initiated, the fuse 110 vaporizes explosively toform the medium to maintain the discharge between the cathode and anode.The cathode and anode are designed so that erosion is essentiallyminimized due to substantially non-eroding material compositions such astungsten. The pressure and temperature inside the bore 120 rapidlyincrease due to the fuse wire and electrode vaporization and subsequentJoule heating. The resulting plasma 128 (FIG. 9) reaches a temperatureof approximately 30,000 degrees K, and rapidly vaporizes the ceramicmicroparticle powder which is inside the cylindrical bore. Due to thelarge favorable pressure gradient inside the device, which producesin-bore velocities on the order of several kilometers per second, thevapor rapidly accelerates through the bore which confines the plasma andforces the plasma to exit from both ends 130.

Some of the micron-sized particles entrained within the plasma 128 willexit as superheated vapor. Because of the different individual particlesize distributions and their corresponding residence times, some of thevapor will dissociate and ionize inside the bore and contribute to thedense metal vapor plasma. This part of the process is referred to asdirect gas phase conversion of ceramic powder to nanocrystallineceramic.

This mechanism is inherently more efficient since energy is not wastedin the subsequent dissociation and ionization of the superheated vapor.In addition, vaporization of micron-sized particles is extremelyefficient. The enhanced efficiency is derived primarily from the greatersurface to volume ratio of the micron-sized particles compared topolycrystalline planar (flat) surface of the bore. This allows for moreefficient transfer of energy and hence more effective vaporization.Other, more subtle, thermophysical processes include nearlyinstantaneous thermal shock fragmentation of the micron-sized particles.In addition, because of the design, the bore wall consisting ofpolycrystalline ceramic erodes due to ablation which is produced by acombination of surface heat transfer and mechanical shear forces. Mostof the resulting ceramic vapor will be entrained in the plasma where itdissociates and ionizes to contribute to the dense metal vapor plasmainside the bore. However, some polycrystalline ceramic vapor will exitas superheated vapor to produce nanocrystalline ceramic by direct gasphase condensation.

As the process continues, the pressure and temperature rapidly increaseinside the discharge bore due to the vaporization and subsequent Jouleheating creating a Raleigh flow which exits from both ends of the tube.The exit velocity is the local sound speed which is several kilometersper second and the temperature and pressure are on the order of 30,000degrees K and 100 atmospheres, respectively.

As shown in FIG. 10, the plasma jet 132 then exits into the reactor 140in the form of a highly underexpanded supersonic plasma jet. The plasmajet expands, entrains and mixes with the reactor gas 142. Because of therapid cooling due to the isentropic expansion, agglomeration is avoidedand the particles condense to form nearly mono-dispersed nanocrystallineceramic 144. This mechanism is a unique feature of the process. Thereactor vessel is normally at or near atmospheric pressure and is filledwith a suitable gas for the reaction. Gases would include but notlimited to air, oxygen, nitrogen, ammonia or a combination there of.

FIG. 11 shows another embodiment of the dual bore device. In thisdesign, a conductive insert 160 is placed in the center of the bore 162.A fuse wire 168 connects the insert to both the anode 164 and thecathode 166. The process is initiated as before, however two sparks arecreated; one from the cathode to the insert and one from the insert tothe anode. The insert can be designed such that it erodes and/or ablatesto sustain the discharge. It is manufactured from a material that willreact with the reaction gas to provide the desired ceramicnanocrystalline particles. It may also contain micron particles that arevaporized to provide more efficient conversion of material. The benefitof this design is that the electrodes can be designed so that they donot wear out and the insert can be designed to be the consumable.Replacing the electrodes can be time consuming and complex since highvoltage connections must be broken to replace them. In contrast, theinsert can be replaced easily since there are no electrical connections.Also, since the explosive forces balance, the need for a high pressureseal is eliminated.

FIG. 12 shows yet another embodiment in which the insert 160 of FIG. 11is replaced with an encapsulated material. An outer coating 180 is madeof a material that will instantly vaporize, such as aluminum orcellulose, and will not contaminate the production. Inside theencapsulation 180 is micron particles 182. The micron particles aretypically the same material as that being produced, or material thatwill react with the gas to produce the nanocrystalline ceramic, or acombination thereof. A fuse wire 184 is fed through the capsule andconnected to the anode 186 and cathode 188 to initiate the discharge.This design allows for efficient conversion of material by directvaporization of the micron particles. It also eliminates the need forthe high pressure seals.

For the production of aluminum nitride (AlN) in the device of FIG. 12,the fuse, cathode and anode would be aluminum. The micron-sizedparticles would be aluminum nitride, and the bore wall would be composedof aluminum nitride. The reactor gas would be nitrogen or ammonia at ornear atmospheric pressure. Similarly, for the production of titania(TiO₂), the fuse, anode and cathode would be composed of titanium. Themicron-sized particles and the bore wall would be titania. The reactorgas would be air or oxygen and an inert gas.

The HEW (hybrid exploding wire) device as described in this document isdesigned to operate in a pulse mode with electrical energy supplied by asuitable power supply. Continuous or high repetition rate operation ispossible but would require active cooling of the device. For thepreferred embodiment, a capacitor bank supplies the energy. Theelectrical energy is stored in a capacitor bank that is charged to adesired voltage, V₀.

Referring now to FIGS. 13 and 14, another embodiment of a dual bore ordual jet electrothermal gun 201 is shown. Dual jet electrothermal gun201 comprises a barrel 202 with a cylindrical bore 203, a cathode 204 atone end, an anode 206 at the opposite end, and a central electrode 208centrally located in bore 203. There are no mechanical electrical (i.e.,direct) connections to central electrode 208. FIG. 14 illustratesoperation of dual jet electrothermal gun 201. A first pulsed electricarc 220 (depicted by heavy dotted lines) passes from cathode 204 to theanode end 210 of central electrode 208. Simultaneously, a second pulsedelectric arc 221 passes from anode 206 to the cathode end 211 of centralelectrode 208. Simultaneous electric arcs 220, 221 generate simultaneoushigh-temperature, high velocity plasma jets 230, 231 in the same manneras the single plasma jet generated in the prior art. The simultaneousplasma jets 230, 231 are introduced into an appropriate reactoratmosphere to synthesize materials as in the prior art.

FIG. 15 is a schematic illustration of reloading central electrode 208of dual jet electrothermal gun 201 after central electrode 208 has beencompletely consumed. Automated loading arm 240, 240′, 240″ picksreplacement central electrode 208′ from storage magazine 242 and insertsreplacement central electrode 208′ into bore 203, then retracts to avoidthe blast of the plasma jets described above for FIG. 14. In the priorart, automated reloading would be a much more complicated and timeconsuming process because of the necessity of removing the stub end ofthe depleted electrode, the necessity of sealing the new electrode, andthe necessity of pre-loading the seal. In the prior art, the stub end ofthe electrode would be wasted. In the present invention it is necessaryonly to locate the new central electrode 208 in bore 203. In the presentinvention, the pressure differential to drive the backward flow ofmaterial around central electrode 208 has been drastically reduced oreliminated. In the present invention, even if plasma leaks aroundcentral electrode 208, the adverse consequences of the leak are smallbecause the leaking plasma enters the plasma jet on the opposite side.

Simplifying the reloading procedure of an electrothermal gun forsynthesizing materials increases the rate of production, decreasingproduction costs. The need for sealing the breech of an electrothermalgun each time a new breech electrode is reloaded slows production. Also,in a conventional electrothermal gun, each time a new breech electrodeis reloaded, care must be taken to make good mechanical electricalcontact between the breech electrode and the bus which supplies currentto the breech electrode, good contact necessitated by the high magnitudecurrents employed to power electrothermal guns. To facilitate initiationof the arc, it is sometimes necessary to use a fuse wire between thebreech electrode and the muzzle electrode. Since a fuse wire needs to bereplaced for each shot, this can also complicate and slow the reloadingprocedure.

An electrothermal gun designed to fire two opposing jets simplifies andspeeds reloading and eliminates the need to seal the breech of the gunand eliminates the need to make a mechanical electrical contact to thebreech electrode. The opposing jet electrothermal gun has no sealedbreech and no breech electrode. The bore is open at both ends anddemonstrates geometric, two-fold symmetry (i.e., the gun isgeometrically symmetrical about the central plane perpendicular to theaxis). The gun has three electrodes including one at each end and acentral electrode which fills the bore. In operation, the electrothermalgun has two arcs. One arc is initiated from the cathode (located at oneend) to the near end of the central electrode.

Simultaneously, an arc is initiated from the other end of the centralelectrode to the anode, located at the opposite end from the cathode.These arcs produce two opposing high velocity plasma jets, one out ofeach end. To synthesize materials, the jets are directed into anappropriate atmosphere, as with prior single jet electrothermal guns.There is no pressure differential between the ends of the centralelectrode, and therefore, no tendency for plasma to flow past thecentral from one barrel to the other.

Loading is accomplished by pushing the central electrode into the bore.This operation is easily automated since the necessity for makingmechanical and electrical contact with the central electrode has beeneliminated, and the necessity of sealing the central electrode has beeneliminated. A fuse wire can be attached to the central electrode so thatthe fuse wire protrudes from each end of the gun after loading. Theoperating cycle of the gun can be easily designed so the centralelectrode is completely consumed so that no electrode stub needs to beremoved as part of the reloading cycle. The anode and cathode arelocated at the ends of the gun, have relatively large capacity and lowconsumption, and therefore require infrequent reloading.

A portion of the barrel of the electrothermal gun is consumed each timethe gun is fired, more or less depending on the composition of thebarrel. Tight fit of the central electrode to the bore is not absolutelynecessary, but a slightly larger diameter central electrode can bereloaded each time to compensate for barrel consumption. Alternatively,a jacket can be provided with each electrode to shield the barrel sothat the barrel diameter does not gradually increase as the gun is used.The disadvantage of using such a jacket is that the used jacket must beremoved and collected each time the gun is reloaded.

Although the invention has been described with reference to specificembodiments, this description is not meant to be construed in a limitingsense. Various modifications of the disclosed embodiments, as well asalternative embodiments of the invention, will become apparent topersons skilled in the art upon reference to the description of theinvention. Those skilled in the art recognize that the process operatesin a combined mode. For example, nanocrystalline alumina is produced ina chemical reaction by the conversion of solid aluminum bar into tonanocrystalline alumina particles in a reaction with gaseous oxygen.Also, solid microphase material is converted to nanophase powder with nochange in chemistry, for example the conversion of solid alumina ceramicbore into nanophase alumina particles by the ablation process. In bothcases, there are no byproducts produced. In addition, reactions go tocompletion in approximately one millisecond which produces a gassuspension of nanocrystalline particles. The nanocrystalline particlescan then be removed from the suspension. It is therefore contemplatedthat such modifications can be made without departing from the spirit orscope of the present invention as defined in the appended claims.

What is claimed:
 1. A system for producing powder, comprising: a reactorvessel having an interior cavity with two open-ended exit ports; ananode mounted adjacent to one of the exit ports; a cathode mountedadjacent to the other of the exit ports; and wherein at least portionsof the anode and the cathode are vaporized for generating powder viaboth exit ports.
 2. The system of claim 1 wherein the interior cavity ofthe reactor vessel is a cylindrical bore and the exit ports are locatedat axial ends of the bore.
 3. The system of claim 1 wherein the anodeand the cathode are formed from the same material.
 4. The system ofclaim 1, further comprising microparticles located within the interiorcavity of the reactor vessel.
 5. The system of claim 1, furthercomprising an automated loading mechanism for automatically reloadingthe reactor vessel.
 6. A system for producing powder, comprising: areactor vessel having an interior cavity with two open-ended exit ports;an anode mounted adjacent to one of the exit ports; a cathode mountedadjacent to the other of the exit ports; and production means forgenerating powder via both exit ports in response to a discharge.
 7. Thesystem of claim 6 wherein the production means includes ignition meansassociated with the reactor vessel for initiating the discharge therein.8. The system of claim 6 wherein the interior cavity of the reactorvessel is a cylindrical bore and the exit ports are located at axialends of the bore.
 9. The system of claim 7 wherein the ignition means isa fuse extending between the anode, the cathode, and the productionmeans.
 10. The system of claim 6 wherein the anode and the cathode areformed from the same material.
 11. The system of claim 6 wherein theproduction means vaporizes to force plasma out of both exit ports. 12.The system of claim 6, further comprising microparticles located withinthe interior cavity of the reactor vessel.
 13. The system of claim 6wherein the production means is a conductive insert.
 14. The system ofclaim 7 wherein the production means is a conductive insert locatedapproximately in a center of the interior cavity of the reactor vessel,and wherein the ignition means is a fuse.
 15. The system of claim 6wherein the production means is an encapsulated insert.
 16. The systemof claim 7 wherein the production means is an encapsulated insertlocated in approximately a center of the interior cavity of the reactorvessel, and wherein the ignition means is a fuse.
 17. The system ofclaim 6 wherein the production means is an electrode located in theinterior cavity of the reactor vessel.
 18. The system of claim 6,further comprising an automated loading mechanism for automaticallyreloading the production means in the reactor vessel.
 19. Anelectrothermal gun for producing nanoscale particles, comprising: areactor vessel having a cylindrical bore with two open ends and areactor chamber at the ends containing a reactor gas; an anode mountedto one of the ends of the bore; a cathode mounted to the other of theends of the bore; ignition means associated with the reactor vessel forinitiating a discharge therein; and production means located within thebore of the reactor vessel for vaporizing in response to the dischargeand forcing plasma to exit from both ends of the bore to generatenanoscale particles in the reactor chamber.
 20. The electrothermal gunof claim 19 wherein the production means is a fuse sheath extendingbetween the anode and cathode.
 21. The electrothermal gun of claim 19wherein the anode and the cathode are formed from the same material. 22.The electrothermal gun of claim 19, further comprising microparticleslocated within the bore of the reactor vessel.
 23. The electrothermalgun of claim 19 wherein the production means is an insert locatedapproximately in an axial center of the bore, and wherein the ignitionmeans is a fuse.
 24. The electrothermal gun of claim 23 wherein theinsert is conductive.
 25. The electrothermal gun of claim 23 wherein theinsert is encapsulated.
 26. The electrothermal gun of claim 23 whereinthe insert is an electrode.
 27. The electrothermal gun of claim 19,further comprising an automated loading mechanism for automaticallyreloading the production means in the reactor vessel.
 28. A method ofproducing powder, comprising: (a) providing a reactor vessel with acavity having two open-ended exit ports, an anode, and a cathode; (b)applying a voltage across the anode and cathode to initiate a discharge;then (c) vaporizing material in response to the discharge to form aplasma that exits both exit ports to generate powder.
 29. The method ofclaim 28 wherein step (a) further comprises providing microparticles inthe cavity, and wherein step (c) further comprises vaporizing andconverting the microparticles into nanosized particles.
 30. The methodof claim 28 wherein step (c) comprises vaporizing and convertingportions of the anode and the cathode into powder.
 31. The method ofclaim 28 wherein step (c) comprises vaporizing a production materialinto powder.
 32. The method of claim 28 wherein step (c) comprisesablating a conductive insert located approximately in a center of thecavity into powder.
 33. The method of claim 28, further comprising thestep of automatically reloading production material in the reactorvessel.